1. Field of the Invention
This invention relates to a method and apparatus for composting. More particularly, this invention relates to continuous and cost-effective composting of large quantities of waste material while maintaining maximum control over the reaction parameters by using closed loop air circulation with an external air manifold.
2. The Prior Art
General methods and apparatuses for composting have existed many years. Composting may even be accomplished without any particular apparatus at all. For instance, windrows can be used. Windrows is the composting of a material by laying it out on a field and periodically turning it over with a tractor However, windrows suffer from a number of deficiencies. First, windrows are highly susceptible to adverse weather conditions. Furthermore, the biological and chemical makeup of the material to be composted cannot be assayed and used to adjust the composting parameters. Mixing of the windrowed material may only be accomplished by manually overturning the windrows. Manual overturning often leaves partially composted material in a non-homogenous state. This non-homogeneity leads to non-uniform temperature distribution as well as anaerobic pockets in the material. These pockets create the obnoxious odors associated with open-air composting methods. Therefore, a need exists for an improved composting method which overcomes the deficiencies of windrowing.
Converting sewage sludge into usable humus fertilizer is an environmentally sound goal. This conversion is accomplished via aerobic stabilization and rotting, i.e., composting. In actual use, static methods of composting (such as windrowing) remain unsatifactory on both the technical and economic level. The dissatisfaction has lead to efforts to develop machine composting.
Since the 1970s, composting has become an important method for stabilizing and processing municipal sewage. See EPA, Summary Report on In-Vessel Composting of Municipal Wastewater Sludge, Risk Reduction Engineering Laboratory, Center for Environmental Research Information, September 1989. The technology has developed extremely rapidly, from less than 10 facilities in 1975 to nearly 200 under design or in operation in 1989. Because of odor, labor, and materials-handling problems, designers are producing composting systems built to contain the materials within a vessel. Municipalities continue to face serious problems in dealing with odors, removing moisture, handling the materials in the system and marketing the product.
A general composting process begins with the mixing of sludge cake, amendment, if any, (e.g., sawdust) and recycling it in an aerated reactor. Air is diffused into the reactor for temperature control, moisture removal and biological metabolism. Air from within the reactor is then exhausted to an odor treatment system before being dispersed into the atmosphere. After a desired detention time within the vessel, the material is removed from the reactor for further curing/storage.
Composting occurs in multiple stages. The first stage is a high-rate phase. This phase is characterized by high oxygen uptake rates, high temperatures, rapid degradation of biodegradable volatile solids and high odor production. The second stage is a curing phase. This phase is characterized by lower temperatures, reduced oxygen uptake rates and a lower, but significant, potential for odor production.
In machine composting methods, the user can control mixing, ventilation, oxygen supply, moisture content and temperature to more reliably, rapidly and economically transit the two phases and, therefore, perform composting. As noted above with respect to windrows, the major problem with composting is the formation of anaerobic zones as a result of insufficient mixing. Despite numerous efforts, none of the existing machine solutions provides a truly simple, economical, elegant solution to the general problems associated with composting.
Other in-vessel composting apparatuses solve, at least to some degree, some of the problems of windrowing. Salvageable materials in the municipal waste are manually, pneumatically, mechanically, or electromagnetically removed. The remainder is comminuted and any paper and film plastic is removed and burned. The heat from burning is used to evaporate water from the sewage sludge or for drying the compost. The remaining waste is then deposited in a series of 1/2" thick layers in a composting tank. Sewage sludge is added. Oxygen-enriched air is introduced through a false bottom to accelerate aeration. An agitator is used to mix the compost and accelerate decomposition. The compost is finally dried with hot air, ground, and bagged as fertilizer. However, such devices do not use a rotating vessel. The lack of rotation of the entire vessel necessitates the use of an external agitator. The likelihood of anaerobic pockets is substantial. Furthermore, it is unlikely that homogenous aeration will occur considering the density of partially composted materials.
Some in-vessel composters employ air tubes extending radially from a central axis inside of a vessel, thereby delivering air to the periphery of the enclosure as the vessel rotates. Because the tubes encounter resistance from the material being composted, they must be made of stainless steel, or other strong and expensive materials. Such composters have the disadvantage of the tubes breaking when forced against large clumps of compost, which then requires time consuming and expensive repairs.